The structure of the transverse tubular system in ventricular and atrial myocytes of the rat was investigated by examining sections stained with the osmiumferrocyanide method. Ventricular myocytes contained longitudinally and transversely oriented tubules which represent invaginations of the plasma membranes and form orderly latticeworks. In atrial myocardium these structures are poorly developed and more pleomorphic. This stain reveals the transverse tubular system to be more extensive and complex than is evident from examination of routinely stained preparations.